The present invention relates to monitoring the temperature of the LED junction in situ and adjusting the current driving the LED according to the determined temperature of the LED.
In various LED lighting applications, determining and maintaining LED junction temperatures during operation is necessary for ensuring reliability and performance of the LED lighting applications. Typically, in order to monitor LED temperature, dedicated temperature sensing diodes or thermistors are placed as physically close to the LED lamp as design constraints permit but external to the LED. For example, discrete precision silicon temperature diodes or thermistors (negative temperature coefficient resistors) are placed in proximity, external to the LEDs, and used to “estimate” the temperature of the LED junctions. However, such external temperature sensors can only measure the ambient temperature of the LEDs and cannot measure the temperature of the internal junction of the LEDs themselves. In LED applications, the conventional approach is to position the external temperature sensors as close to the solder or thermal pad point of an LED as possible, so that the measured temperature is the LED's local ambient temperature, circuit board temperature, or external solder point temperature. However, the measured temperature is still not the actual LED PN junction temperature. Thus, these conventional approaches are not capable of sensing the actual LED PN junction temperature, because they are external to the LED. Especially, at higher operating currents, the actual LED junction temperature may be significantly different from the ambient temperature, and thus conventional LED temperature sensing techniques can be largely inaccurate. In other words, these external temperature sensors have the disadvantage that they cannot measure the critical junction temperature of the LED itself. This limits the ability to accurately monitor the real time self-heating effects of the LED.
Furthermore, conventional LED applications typically use pre-characterized thermal information to “pad” the measured temperature of the external point sensed by the external temperature sensor to account for “estimated” variations at the LED junction. This “padding” scheme has the disadvantage that system cost may increase as much as 20% as more LED lamps are required in these “padded” designs to meet the needed illumination specifications.
One approach, known as the CTAT (Complementary To Absolute Temperature) technique, uses the LED itself as the temperature sensing device with the specific LEDs pre-characterized and calibrated extensively under controlled conditions to determine the “K” factor (i.e. the change in LED forward voltage, VF, over temperature, at a constant forward bias current, IF). The CTAT approach is based on the principle that the LED forward voltage, VF, has a decreasing rate with increasing temperature. However, the CTAT technique requires both a known starting temperature point and the pre-characterized CTAT temperature coefficient (i.e. “K” factor) a priori in order to calculate a new temperature point. Thus, CTAT itself is not a solution for in situ sensing of LED junction temperature, since it is not possible to control the starting temperature point of LED applications in actual use.
Another method of temperature measurement for diodes is called the PTAT (Proportional To Absolute Temperature) technique, which resolves the issue of determining the starting temperature. The PTAT method is the basis for band-gap regulator circuits, and takes advantage of the fact that the difference in the forward voltages (VF2−VF1) across a PN junction of a diode taken at two different forward currents, IF2 and IF1, is directly proportional to the absolute temperature. The PTAT approach works well for silicon based temperature diodes with near ideal characteristics. However, as LEDs do vary considerably from the ideal diode properties, use of the PTAT approach to sense the temperature of LEDs require extensive up-front calibration, which makes the use of the PTAT approach itself impractical for LED temperature sensing during “real time” operation.